The present invention relates to a method of forming a protective film for a magnetic recording medium, a protective film formed by the method, and a magnetic recording medium having the protective film.
Hard films made of various substances are used for coating a slide-resistant member or a wear-resistant member. Among them, a diamond like carbon (DLC) film, a hard coating film made of carbon, is known. The DLC film exhibits good surface smoothness and great hardness, and is suited for a surface coating film. The DLC films have conventionally been formed by a sputtering method, a plasma CVD method and the like.
Magnetic recording media today have a protective film of a DLC film formed on a magnetic recording layer by a plasma CVD method. The purpose for the protective film is to protect the magnetic recording layer against corrosion and damages due to contact or sliding with a magnetic head.
The recording systems of magnetic recording media have been changing in recent years from a longitudinal system to a perpendicular system, the latter allowing higher recording density. In the magnetic recording media in the perpendicular system, like in the longitudinal system, the protective film needs to be as thin as possible to attain high recording density while holding the performances of the durability and corrosion resistance.
In addition to the durability and corrosion resistance, a protective film further needs a good bonding characteristic with a lubricant film formed on the protective film and suppression of adhesion of contamination gases. Accordingly, the protective film needs to have an optimal surface composition. A DLC film is known to have a relatively active surface and to be liable to be adhered thereto by acidic impurities, especially sulfur-containing impurities, although it exhibits good durability and corrosion resistance.
In view of this issue, Japanese Unexamined Patent Application Publication No. H09-128732 discloses a method in which a nitrogenized amorphous carbon (a-C:N) film is deposited by a sputtering method on a DLC film formed by a plasma CVD method. The a-C:N film is formed by sputtering a carbon target in a mixture gas containing nitrogen. The method of the document intends to simultaneously attain: (a) compensation for mechanical strength that is insufficient with solely the a-C:N film, by laminating with a DLC film, (b) suppression of adhesion of contamination gases by providing an outermost surface of the protective film with an a-C:N film which contains smaller amount of hydrogen than a DLC film, and (c) good bonding with the lubricant film by virtue of the nitrogen which is contained in the a-C:N film.
Japanese Unexamined Patent Application Publication No. 2001-126233 discloses a method that does not use an a-C:N film and employs direct nitrogenization of the surface of a DLC film formed by a plasma CVD method. The method achieves good bonding with a lubricant film by virtue of the nitrogenization, and at the same time, allows thickness reduction of the protective film owing to absence of an a-C:N film.
The method of Japanese Unexamined Patent Application Publication No. H09-128732 using an a-C:N film, however, suffers from a thickness problem with respect to the protective film due to employment of a lamination, though it sufficiently suppresses adhesion of contamination gases. On the other hand, the method of Japanese Unexamined Patent Application Publication No. 2001-126233 using direct nitrogenization of a DLC film suffers from a problem of insufficient suppression of adhesion of contamination gases due to excessively large amount of hydrogen on the DLC film surface formed by a plasma CVD method, although it is advantageous in thickness reduction of the protective film.
In view of the above, it would be desirable to provide a method of forming a protective film of a magnetic recording medium, the method achieving simultaneously good bonding with a lubricant film and sufficient suppression of adhesion of contamination gases as well as good durability and corrosion resistance to attain reduced thickness of the magnetic recording medium.